The present invention deals with a specific flat-soffit roof-ceiling construction of an original concept and shape. Although some similarities to trusses or tied arches are obvious the present construction substantially differs from them in the manner of how it works bearing the load. First of all, these constructions are intended to solve both the finished ceiling with flat soffit and the roof construction simultaneously. It is intended also to use the wide soffit plate to contribute as a bearing element instead of being passively hung on a truss or an arch.
All the other practical intentions of the present construction include Advantages disclosed by HR-P20000906A that these constructions have when compared to customary roofs and ceilings.
The commonly used prestressing techniques that introduce compressive force into a structural member of a selected geometry cross-section with tendons positioned below the concrete center of gravity would not achieve proper effects when applied to these constructions because of the absence of such an eccentricity. An achievement of upward deflection of the concrete plate would require lowering of the prestressing tendons below the center of gravity of the overall construction which is unacceptable because it would ruin the idea of the flat soffit. The problem is hence focused to find out an adequate prestressing method which may efficiently reduce the large amount of deflections and eliminate or control cracks in concrete which may occur if tension in the soffit plate is allowed. The present invention provides one more efficient method for prestressing constructions having a flat soffit. The present construction also solves the problem of stability of the upper girder against lateral buckling.
The HR-P20000906A application, under the name “Doubly prestressed, composite, roof-ceiling construction with flat soffit for large span buildings” is the most similar known construction. The just mentioned application proposes one efficient method for prestressing of such inverse constructions with a low positioned center of gravity of the cross section and discloses another solution: The wide plate is prestressed once, centrically, before the construction is completed, introducing compression into the soffit plate wherewith the cracks problem in concrete is solved. The construction is then completed and is prestressed once again by means of a steel wedge driven into a special detail positioned at a midspan of the upper girder to achieve an upward deflection of the plate by rotating its ends.
The present invention relates to a very similar but substantially changed construction from the one disclosed in HR-P20000906A. One more additional prestressing is provided. In comparison to the abovementioned innovation the present construction introduces the stiff upper girder with such a design of the cross-sectional shape which is simultaneously rigid and thin-walled, intended to reduce an effective length of the interconnecting pipe-rods compared to considerably stiff steel tubes. Replacement of stiff steel tubes by slender pipe-rods disables transmission of bending moments from the upper girder to the plate and vice versa. The interconnecting pipe-rods are spaced uniformly over the soffit plate to improve the interconnection and uniformity of the plate weight distribution on the upper girder. Hence, the connections between rods and the plate became less rigid so that the prestressing force introduced in the soffit plate causes no considerable bending of rods and enables a larger amount of prestressing to be applied without bending the plate. However, if the centric prestressing of the soffit plate is performed in a small amount it does not significantly influence the deflection of the plate. If, in contrast in contrast, a large amount of prestressing force is applied at a high compression levels, considerable influences are applied to deflections of the soffit plate. It is one important object of the present invention to provide one more efficient manner of prestressing constructions with the flat soffit while not disputing double prestressing as a very efficient method.
The present construction solves the problem of stabilizing the upper girder against lateral buckling more efficiently than the abovementioned application. The space-distributed connecting rods, distributed uniformly over the upper plane of the ceiling plate at certain, determined distances, divide the overall effective length of the upper girder into a plurality of smaller lengths whereby the cross section of the upper girder is of an inverse “V” shape that shortens the effective lengths of interconnecting rods and changes their end conditions, reducing additionally their effective lengths of buckling.